Experimental and Theoretical Investigation of Homogeneous Gaseous Reaction of CO₂(g) + <i>n</i>H₂O(g) + <i>n</i>NH₃(g) → Products (<i>n</i> = 1, 2)

Decreasing CO₂ emissions into the atmosphere is key for reducing global warming. To facilitate the CO₂ emission reduction efforts, our laboratory conducted experimental and theoretical investigations of the homogeneous gaseous reaction of CO₂(g) + <i>n</i>H₂O­(g) + <i>n</i>NH₃(g) → (NH₄)­HCO₃(s)/(NH₄)₂CO₃(s) (<i>n</i> = 1 and 2) using Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy and ab initio molecular orbital theory. Our FTIR-ATR experimental results indicate that (NH₄)₂CO₃(s) and (NH₄)­HCO₃(s) are formed as aerosol particulate matter when carbon dioxide reacts with ammonia and water in the gaseous phase at room temperature. Ab initio study of this chemical system suggested that the reaction may proceed through formation of NH₃·H₂O­(g), NH₃·CO₂(g), and CO₂·H₂O­(g) complexes. Subsequent complexes, NH₃·H₂O·CO₂ and (NH₃)₂·H₂O·CO₂, can be formed by adding gaseous reactants to the NH₃·H₂O­(g), NH₃·CO₂(g), and CO₂·H₂O­(g) complexes, respectively. The NH₃·H₂O·CO₂ and (NH₃)₂·H₂O·CO₂ complexes can then be rearranged to produce (NH₄)­HCO₃ and (NH₄)₂CO₃ as final products via a transition state, and the NH₃ molecule acts as a medium accepting and donating hydrogen atoms in the rearrangement process. Our computational results also reveal that the presence of an additional water molecule can reduce the activation energy of the rearrangement process. The high activation energy predicted in the present work suggests that the reaction is kinetically not favored, and our experimental observation of (NH₄)­HCO₃(s) and (NH₄)₂CO₃(s) may be attributed to the high concentrations of reactants increasing the reaction rate of the title reactions in the reactor

Kinetic and Dynamic Investigations of OH Reaction with Styrene

The kinetics of hydroxyl radical reaction with styrene has been studied at 240–340 K and a total pressure of 1–3 Torr using the relative rate/discharge flow/mass spectrometry technique. In addition, the dynamics of the reaction was also studied using the ab initio molecular orbital method. The reaction was found to be essentially pressure independent over 1–3 Torr at both 298 and 340 K. At 298 K, the average rate constant was determined, using four different reference compounds, to be <i>k</i>_styrene+OH = (5.80 ± 0.49) × 10^–11 cm³ molecule^–1 s^–1. At 240–340 K, the rate constant of this reaction was found to be negatively dependent on temperature with an Arrhenius expression determined to be <i>k</i>_styrene+OH = (1.02 ± 0.10) × 10^–11 exp[(532 ± 28)/<i>T</i>] cm³ molecule^–1 s^–1. Observation of mass spectral evidence of adduct products and their respective fragment ions suggests that the reaction proceeds with addition of the OH to the vinyl carbons of the styrene molecule. Ab initio calculations of both the addition and the abstraction pathways predict that the addition pathways are more energetically favorable because of large exothermicity and essentially barrierless transition state associated with the additions, which is consistent with the experimental observations. Using the styrene + OH rate constant determined at 277 K in the present work, the atmospheric lifetime of styrene was estimated to be 4.9 h